This invention relates generally to improvements in electromechanical acoustic emission detectors or transducers for detecting acoustic emission signals that are emitted from solid objects with those signals being caused by cracking or failure in some way in the solid object of interest. The signals obtained are converted into electrical signals having components of corresponding frequency. The electric signals may be converted into computer readouts, for example, for grading acoustic emission signal being emitted. This invention relates particularly to improvements in such an arrangement wherein a waveguide is utilized for applications where the temperature levels or other conditions involved in the solid object of interest being measured require that the sensor itself and those handling the sensor be spaced from the surface of the object of interest.
That is, the phenomonon of acoustic emission, to which the present invention relates, is concerned with the detection of elastic waves that are emitted from a source within an object and become manifest at positions remote from the source. Such waves are developed, for example, in the cracking of a pressure vessel shell either internally, or on the surface when it is failing or deteriorating. Many times, such pressure vessels operate at extreme high or low temperature levels, and it is necessary to provide a spacing between the sensor and the object in order to protect the sensor from the extreme temperature levels. Other times, the object being examined may be operating at high voltage levels, for example, necessitating that the operator of acoustic emission equipment be spaced from such sources.
Thus, the invention is directed to waveguides which may be used to transmit acoustic signals from the object of interest over a spacing to the sensor, and it is an object of this invention to provide a stable electrical insulation for the operator in high voltage applications, and stable acoustic emission transmission at extreme high or low (i.e. cryogenic) temperature levels, while at the same time not subjecting such applications to dispersive signal propagation and acoustic signal loss. The waveguide serves to transmit signals from the surface of the object of interest to the sensor, and maintain the sensor in a spaced relationship and protected against any undesirable circumstances which may affect the user or the sensor.
In the past, acoustic emission waveguides have been comprised of metal rods, or glass or glass composites for insulation purposes. These have not proved entirely satisfactory because they result in high signal loss from dispersion of the signal in the waveguide, size limitation of the cross section, intrinsic acoustic damping in the prior art material, and acoustic impedance mismatch among the waveguide materials, sensors and structures being tested. Such problems may result in a total signal loss of 10 to 20 dB for a common waveguide of one foot.
With this invention, by contrast, a waveguide is provided which is solid and elongated and provides the desirable electrical and/or temperature spacing necessary from the object of interest to the sensor element while reducing signal loss to around 1 dB for each foot of waveguide. The waveguide is comprised of a member selected from the group consisting of aluminum oxide (alumina) and beryllium oxide. Preferably, aluminum oxide is used because it is substantially less costly. Moreover, aluminum oxide is not electrically and thermally conductive as is beryllium, and its acoustic impedance is in better conformity with that of sensors and routine structures being tested.